At War with the Heart
by BLionQueen
Summary: In war, there is no room for love or understanding between enemies. There is a definite line that separates you from your opponent. But what happens when those lines begin to blur?
1. Chapter 1

**Hello, world! So, I know this is not a popular opinion, but I totally love Muzan! I'm not sure why, but I just love the guy! I think I might have a thing for bad boys... Oh well! Just be aware that this has major manga spoilers. Don't read if you haven't red pass chapter 70. I do not own Kimetsu no Yaiba or its characters. This is purely fanfiction for fun. **

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What would you do for the ones you love? Would you die for them? Kill for them? Would you be willing to make a deal with the devil? For Kamado Tanjirou the answer was simple. He would do __anything__ for his loved ones. He would risk his life and soul to turn Nezuko back into a human.

It was a not-so quite night in the Red-Light District. Tanjirou was buried underneath his futon, trying to cancel out the noises coming from the rooms next to him. The moans and grunts that some of them men and women were making were starting to make Tanjirou feel uncomfortable.

According to the Hostess, every night was a busy night for them. Tanjirou's ear could clearly attest to that. It was way late into the night, the sun would rise in a few hours, yet so many women we're still entertaining men.

Not a single customer had requested for 'Sumiko' tonight, so Tanjirou was ordered to do housework and tend to the women's needs between customers. Once there was nothing left to do around the house, Tanjirou was dismissed and sent to rest. Tanjirou went to his room with the intention to rest his eyes only for a few minutes. He would sneak out of the house as soon as the coast was clear. There was a demon on the loose, and Uzui-san was depending on him and the other two to search for his three wives. He could not afford to sleep for very long, not while people were being threatened by a demon.

That was initially his plan, but…. How can he possibly sleep with all those embarrassing noises!? His virgin ears could not possibly cope.

"Sumiko-chan? Are you awake?" a gentle voice spoke from outside Tanjirou's room. It was one the girls he spoke to earlier. If he remembered correctly, her name was Yukino-san. "You have a guest. She would like to speak with you now."

Tanjirou rose from underneath the plush covers of his futon. "Yes…" Suddenly, a putrid smell of blood and death filled his nostrils. This familiar, overwhelming smell of blood…. It could only belong to one person.

__No! Not a person! This scent… It belongs to HIM! __

The demon slayer rushed to slide the tatami doors open. "You monster!" He expected to come face to face with the devil himself. He did not expect to come face to face with a sinfully beautiful woman like the one staring at him right now. And this smell… It was the same smell of blood and death he detected last time he saw him, but something was different about it this time. __She__ was different.

"Oh, my. When your hostess said that the new girl was rather spirited, I did not expect her to be this excitable." The pale beauty said with a smirk. Her voice smooth and gentle, like that of a noble lady.

"P-Please forgive her abrasiveness, Yukihime-sama! She has not been here very long."

"Even if she hasn't been here very long… You people should at least take the time to teach yourselves some manners! She just accosted Yukihime-sama!" A third voice spoke angrily.

So consumed he was with the source of the putrid smell of blood, Tanjirou did not notice the third woman present. She was fair skinned with yellow hair and light, green eyes. She dressed in a silk blue dress and black gaiters. Her Japanese was impeccable, but Tanjirou could detect a slight accent in her speech. She must be a foreigner from another country. Tanjirou could not possibly guess where this blonde woman was from, he had never met anyone outside from Japan, but he could clearly tell she is not a demon.

"I deeply apologize, Yukihime-sama!" Yukino bowed her head in apology. "If you would please wait in your usual room, I promise I will bring you and your guest our best tea. Sumiko-chan will join you shortly, as soon as I have a talk with her."

"There is no need for that, Yukino. Anne was just being overprotective as usual. I would just like to be alone with 'Sumiko-chan.'"

Tanjirou's hands twitched. Just being around this person made his blood boil and every muscle in his body scream to cut his, or rather her, head off right now. "I-It's alright, Yukino-san." He said through clenched teeth. His glaring eyes never left those red rubies. "I promise I'll behave."

"W-Well, if you insist." Yukino glanced between the two of them nervously. She cleared her throat uncomfortably. "If there is anything you ladies need, I won't be far." Her nervous glance was the last thing she gave them before she turned her back to leave.

Once Yukino was out of eyesight, Tanjirou moved to strike. "You! You can't fool me, Kibutsuji Muzan!" He did not have his sword on him. Instead, he went straight for her neck with his bare hands.

But just before he could make contact with the monster, a sudden hard punch to his open neck left him breathless. It took everything in him to fall on his knees from the sudden loss of breath. Before he could even retaliate, a heeled shoe kicked him in the stomach. Hard. The poor demon slayer landed on his back, several feet away from where he previously stood. The force of the kick alone made him want to spill the contents of his stomach out, but the pain of having his trachea punched kept him from throwing up.

"Try to lay one filthy finger on her again, Scum! The next time you try, I'll break every bone in your body!" The foreign woman shouted. She stomped over to Tanjirou and dug her heeled shoe on Tanjirou's aching stomach. "Where is it!? Your nichirin sword!I won't allow you to speak to Muzan-sama until you hand it over!"

Tanjirou's hands grasped her foot, trying to get the pressure off his stomach. "I won't tell you!"

"He doesn't have it." Muzan said casually as she entered the poorly lit room, closing the doors behind her. "I don't sense the presence of nichirin steel. He's completely defenseless, Anne."

"Y-Your h-human. You know what she is." Tanjirou stated rather than asked. "A demon!"

"Yes. What of it?" She glared at him intensely.

Tanjirou glared at her in return, more out of confusion than anger. "Why!?"

Anne did not bother to answer his question. Instead, she dug her heel in deeper.

"Anne's connection to me is none of your concern, Kamado Tanjirou. You should be more concerned with the reason why I have decided to come visit you tonight." Muzan, like a predator nearing its prey, stalked over to him.

"I don't care why you're here!" He hissed. "I promised myself I would avenge my family! I swore I would lop your head off the next time I saw you!"

Muzan pressed her lips together, trying hard not to burst out laughing. "How interesting! You actually believe you have a chance at accomplishing such ludicrous goals?" She smirked. "You barely could anticipate Anne's movements, and… As you just pointed out, she is merely human. If you cannot even defend yourself against an ordinary human… How can you possibly hope to defeat me?"

All she received in response was a growl and a frightening glare.

"Oh well… I did not come all this way just to watch you humiliate yourself." She elegantly sat beside him. Her knees barely inches away from his face. "I've come here to offer you a proposition that will benefit the both of us."

"I don't make deals with devils!" Muzan's feminine hard grasped his chin. Her sharp nails dug into cheeks, drawing small drops of blood. His face was forcefully turned to make eye contact with her. He hated this creature! There was absolutely nothing that he wanted from her. Other than her death. "Not even to turn your sister back into a human?"

His heart skipped several beats.

"Tell me, Tanjirou… What would you be willing to do for the sake of her humanity?"

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**Review!**


	2. Chapter 2

Don't read if you haven't red pass chapter 70. I do not own Kimetsu no Yaiba or its characters. This is purely fanfiction for fun. 

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"Tell me, Tanjirou… What would you be willing to do for the sake of her humanity?"

Tanjirou stared incredulously at the demon holding his face in a forceful grip. He searched her ruby eyes, trying to detect any falsehood. Usually, Tanjirou could easily tell when someone was lying just by looking into their eyes, but this person, this demon, was extremely hard to read. How could he possibly believe in a single word that she said? How could she possibly believe that he would willingly allow her to be anywhere near Nezuko? For all he knew, this could just be a trap.

"I don't exactly like to kept waiting, Boy." Muzan narrowed her eyes in annoyance as she tightened her grip on his face. "Answer me now, or any hope you have of turning your sister back to a human will disappear along with my patience." She inched her face closer to his, as if searching for something. "Ah, you want to ask me something, don's you? I can see it in your eyes."

Tanjirou winced in pain when the foreign woman, who still had him pinned down like a rat, dug her heel even deeper into his stomach. "I-I can't trust you. You killed my family... Turned Nezuko into a demon..." He swallowed loudly. "W-Why would you help us?"

Muzan huffed. "That's not what you wanted to ask." She finally released his abused cheeks. "No matter. It's not your honesty that I need."

Tanjirou was taken back by her statement. Was he so easy to read?

"I'm going to get nowhere with this wanna-be earnest fool if you keep trying to carve a hole into his stomach, Anne." The demon woman directed her gaze to the blonde human in the room. "I think I've got it from here. The boy is no threat to me. Wait outside."

Anne slowly removed her foot from Tanjirou's stomach. "Muzan-sama, I cannot trust that this boy won't try to contact any one of his friends. If he is here, there is bound to be more of them. There might even be a Pillar present."

"I'm perfectly aware of that, Anne!" Muzan snapped. "Do you mean to insult me? I am perfectly capable of handling one unarmed boy. If you are so concerned about intruders, then stand outside and seal this room!"

Anne's cheeks flared in embarrassment. Her hardened eyes softened in what appeared to be sadness. "I-I deeply apologize, Muzan-sama…" She lowered her head in shame. " I meant no offense. P-Please allow me to be of use to you."

"Then do as you're told."

"Yes.." Anne lowered her head even further. She quietly scurried away and out of the room. Her hands hesitated to close the shoji doors behind her. One glare from Muzan moved her to shut them completely. Like a loyal dog waiting for her master, she cemented herself outside the room.

Tanjirou couldn't help but feel sorry for her.

He turned his attention back to the bane of his existence. "Answer me. Why do you want to help us!?"

"I never said I wanted to help you. I said I wanted to make a deal with you."

Muzan rose from her position next to him. She turned her back to him as she paced around the room. From the corner of her eye, she could see Tanjirou;s eyes following her every move carefully. This brought a smirk to her lips. The boy was right to be weary. It's not everyday when a dangerous predator comes to mingle with the prey. "I have a proposition for you... You give me something I want, and I will help turn your sister back into a human."

Tanjirou slowly stood up. "What do you want from me? If you want me to kill someone, forget it! If you want me to betray my comrades, I'll never do that!" He clenched his fists in anger. "Nezuko would never forgive me if I did those things for her sake."

Muzan stopped in her tracks. Tanjirou could smell her hesitation. What was this? Was the great Kibutsuji Muzan really afraid of answering a human's question? Such a thing almost made her look human...Almost...

As quick as the tender moment came, it disappeared.

Suddenly, Tanjirou felt a strong force push him. He feel backwards, slamming hard against the hard wood-flooring. Tanjirou's instincts told him to get up. He should bounce back. Raise his guard. Do something! Sadly, his human body could not keep up with the speed of a demon. In just a matter of seconds, the demon monarch rough snatched his obi, tied his wrists above his head, and pinned him down. Muzan straddled him with a humorous smirk.

"Now, who said I wanted you to kill someone?" She giggled. "As tempting as that is... That is not what I want from you..."

"Then, what!?" Tanjirou snapped. His patience had finally run dry. He couldn't bare to be in this monster's presence anymore. His hands itched for his sword. His entire body felt like it was on fire right now. There was nothing more than he wanted to do right now than to push her off of him, and run his sword right through her chest.

"What do you want, Muzan!?" His intense glare almost made Muzan think twice about her choice.

__It will only be for a moment...Then reward will be well worth it...__

She lowered her face to his ear. Her hand parted his yukata, feeling the firm chest underneath her. "I want..."Her hand traveled further down. "Your seed."

Tanjirou's whole body tensed when he felt a cold hand grasp his penis. "Whaaaaat!?"

Muzan's mad laugh filled the room. "Look at you! You're as red as tomato! Are you perhaps a virgin, Tanjirou?" She tilted her head in amusement. "Oh! But, look here!" Her hand tugged at his semi-hard penis. "You're reacting so quickly. Being surrounded by so many beautiful women. Having to hear them moan and scream as they pleasure their clients... Perhaps you were already begging to get aroused just from the sounds around you..." She giggled once more. "Such is the power of youth! This should be rather easy, after all."

"G-Get of-" Tanjirou's protests were cut off with an open-mouthed kiss. The flustered boy stiffened when the suprisingly-soft lips engulfed his. His hips tensed when the hand holding his member began to stroke him. His muffled protest were ignored.

A long, slithering tongue entered his mouth. It must be longer than the average tongue. Tanjirou could feel it touching the back of his throat. He tried to fight her back with his own tongue, but she was more skilled at this sort of thing. He could try to bite her, but he couldn't take the risk of accidentally ingesting her blood. All he could do was endure this long, deep kiss.__I can't breathe!__

Muzan broke the deep kiss. A trail of saliva forming between their parting lips. "You really are just an innocent little virgin..." She licked her lips. Her hand on his member began to move faster, pumping him. Tanjirou held back his moans, but then she began to squeeze his tip every time she pumped upwards. __What is it!? It feels... Good!__

Her unoccupied hand worked on removing her silky, expensive clothing. "Relax... This will be one of the most pleasurable experiences of your short life..."

Tanjirou was breathless. This demon killed his familhy, turned his beloved sister into a demon, stole his frits kiss, and now she wants to rob him of his innocence? How much more could this demon take from him? __I want...Your seed.__

Tanjirou was brought back from his thoughts when the hand on his member squeezed and tugged him. He yelped in surprise. His member was fully erect now. He had the animalistic urge to move his hips. He was somewhat relived when the hand on his cock released him. Her small hands parted her yukata, revealing her creamy, white skin.

Tanjirou's cheeks flared. He should be trying to look away, but his eyes were glued to her figure. Her skin looked flawless and soft. Her breasts were supple with two pink nipples staring back at him. His eyes subconsciously traveled from her soft shoulders to her soft breasts, small waist, wide hips, and all the way down to the small patch of black curls covering her womanhood.

His cock twitched in anticipation.

"How cute. Even though you're still a virgin..." The devious temptress lowered her face to his neck. "You respond so eagerly."

"Ah!" Tanjirou winced, half in pain and half in pleasure, when she nipped his neck. "S-Stop!"

"Do you really want me to?" She whispered softly as she continued to trail open mounted kisses down his neck and chest. She nipped the soft flesh with her sharp canines, sucking on it slowly. She could hear his heart beating loudly against his chest. His labored breathing and clenched jaw were signs of his retrain, but Muzan knew that the poor virgin could not possibly last long. "Don't worry. If all goes well, you and I will never do this sort of thing ever again." She went even lower down his body until she was mere inches away from his quivering member. "So, we might as well as enjoy it while it lasts..." Her long tongue darted out of her red lips to lick the precum running down his shaft.

"Hn!" His hips jerked up in response to her licking.

Muzan laughed at his reaction. She went reached behind her to undo her hair bun, releasing her thigh-length curly, black hair. Tanjirou wanted to run his fingers through it.

Her hand grasped his cock again. She aligned herself so that his hardened member touched her sensitive folds.

Tanjirou's eyes widened when he felt the heat of her wet lips. "Stop! I-I don't wan-"

"Don't want what? Don't want to help your sister? Tanjirou... Earlier you said that you would never kill anyone, even if it meant helping your sister. Well, I'm not asking for you to kill anyone... I'm asking you to create life... With me..." She began to drop her hips. "Inside of me..." She began to take him in, inch by inch. Her red eyes stayed glued on his widened ones. She stilled when she finally took him all in, from tip to base.

Tanjirou had never felt anything like this before. Her moist walls engulfed his sensitive cock. The sensation of being inside something so warm and inviting felt amazing. It felt so sinful, but so exhilarating! Just the feel of her muscles squeezing him felt like euphoria. He never knew that sex could feel this beautiful.

Muzan lifted her hips until only the tip of his penis was inside her. Then, she slammed herself down on him.

"Ah! What is this!?" He threw his head back in pleasure as the woman on top of him continued dropping and rocking her hips.

Each time she nearly unsheathed him, she would squeeze him. Her breasts began to bounce when she picked up her speed. She smirked when she felt his hips began to thrust upward, tryin to match her rythm. "Good boy! Make sure not to hold back!" She lowered her head to his chest. Tanjirou grunted when he felt her bite his nipple. By now, he had lost all sense of control. All he could focus on was trying to thrust upward into her hot cavern. His mind blanking from everything except the extreme pleasure he was feeling right now.

Muzan caught his parted lips in another deep kiss. This time, he responded with his own tongue play.

Tanjirou gasped when she began to message his balls. His penis pulsed in response. His balls tightened. He could feel something begging to build inside him. It felt like he was ready to burst. "I-I..."

"Cum, Tanjirou!" She shouted, quickening her pace. "Give me your seed!"

Her words sent him to the edge. With small burst of strength he broke free from his restraints. His hands grasped her hips to still them. His own hips jerked erratically as he released his seed inside of her. Tanjirou grunted like a wild animal with closed eyes. Muzan moaned as his hot cum filled her waiting womb. She caressed his face, almost lovingly. When he finished, she disconnected them slowly and gave him a tender kiss on his forehead.

"Good boy. I have to go now, but I promise I'll keep my word to you... If all goes as expected, of course..." She placed a soft hand over his eyes, coxing him to close them again.

Her smile was the last thing he saw before he passed out.


	3. Chapter 3

**I'm baaaack, my beautiful readers! Your reviews would be greatly appreciated. **

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Muzan stared at the unconscious human boy underneath her. Such a peaceful expression on his face. Such pure innocence. Hard to believe that such an innocent boy had just lost his virginity to a demon. _I wonder if he'll even remember all of this_. She huffed in amusement. He might try to forget, to pretend that he never enjoyed this interaction, but Muzan will never cease to be a part of this boy's life. Not after tonight.

Muzan unsheathed the unconscious boy's member. She could feel the warm trickle of his cum dripping down her thighs. She could sense that Anne was still waiting for her outside the shoji doors, ever the loyal servant. Her hands worked quickly to dress herself. She paid no heed to the boy as she headed towards the doors. When she pushed the shoji door aside, Anne turned to face her.

"We're done here, Anne." Muzan said as she walked pass her human servant. "Pay the house their dues, and let's get the hell off this pitiful island."

"Yes, Muzan-sama." Anne made haste to do as her master ordered.

Muzan, not wanting to stay any longer inside the brothel, decided to head out without Anne. She stepped outside in hopes of getting some fresh air, but that desire was quickly squandered when she took her first breath outside. Muzan wrinkled her nose in disgust. The air was filled with human stench. Sweat, lust, and alcohol filled the air, making it almost impossible for someone with senses like Muzan to enjoy clean air.

_There used to be trees here..._ The demon mused, remembering a time when trees and lush grass used to cover this area. _Humans. All they do is pollute and destroy everything for their own selfish reasons... Their greed..._

Muzan felt the air change. A dark presence began to emit from the brothel behind her. The air was suddenly overwhelmed with the smell of blood. "Daki…"

"Muzan-sama!" Anne rushed out from the brothel to her master. "This presence... It's one of the Upper Moons."

"I know. Upper Moon Six, Daki. She must be after this brothel's Oiran." Muzan shrugged her shoulders. "That girl has always had a habit of eating her competition. She hates losing." She turned to walk away. "This does not concern us. I got what I wanted from the boy. Now, I just want to go home."

Anne nodded. Not even bothering to give the brothel another glance as she hurried to join her master. "Should I tell Fuyumi to keep her distance while the Upper Moon is around?"

"No. Tell Fuyumi to fallback completely. I don't want anymore surveillance on them. Once Tamayo receives the last ingredient, my business with the Kamado siblings will have come to an end."

"Yes, Muzan-sama."

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t was the sound of someone rapping the doors to his bedroom that woke him. Tanjirou's eyes slowly opened. He felt so different, like something inside had changed. His mind felt foggy. _What happened?_ The boy looked around the dark room. The musky scent of s hung heavy in the air. _Oh..._ Finally, the events of a few minutes ago began to come back to him. _Oh gods...What have I done!? _

_How could I have done that!? With him!? I hate him! He killed my family! He turned Nezuko into a demon! What kind of Demon Slayer am I!?_ Shame, disgust, and self-loathing weighted on heavily on him. Still, the promise of their deal was engraved in his mind._ If he's telling the truth, than Nezuko will finally become human again, but how can I trust that that monster will keep his word? _

"Sumiko-chan, can you come outside please?" A soft voice from outside the room caught Tanjirou's attention. It was one of the cleanning girls from the house. She stood outside, rapping the door for the tenth time. "I know you just finished servecing a customer, but could you please help us out here. We're really short-handed right now."

"I-I'll be right out!" Tanjirou jumped to her feet. He rushed to open the door. The young boy froze in front of the door. Realizing that his robe was open, he blushed. He made quick haste to make himself decent.

_Keep it together, Tanjirou! What's done is done! This doesn't change anything... Focus on the mission! Avenge your family! _

_Kill Muzan!_


	4. Chapter 4

**Hello, dearies! Here's another chapter! **

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Tanjirou was never known to be a skilled liar. Even a complete stranger can tell when the honest boy was keeping a secret. He guessed it had something to do with how he was raised. His parents were honest, good-natured people, and they raised their son to be the same. That is why it took a great deal of effort for Tanjirou to open the shoji doors and pretend like nothing life-changing had happened,

"Sumiko-chan!" The young courtesan greeted the disguised boy. "Thank goodness you answered. I'm terribly sorry to bother you after you just finished servicing a customer, bu- What's wrong?"

Apparently, Tanjirou's attempt to hide his distress had failed. His eyes must have given something away, because now the courtesan was staring at him with concerned eyes.

"Did something happen with Yukihime-sama?" She questioned. "I would be surprised if something did. Yukihime-sama is a favorite here in the brothel. She's usually very kind and generous. None of the courtesans here have ever complained about any offensive behavior. Unless..." She gasped, clutching Tanjirou's arm softly. " Could it be... that hat was your first time with a woman?"

"U-Uh..."

The courtesan giggled. "Oh, you silly girl! There's absolutely nothing to be worried about. No one here will judge you for enjoying the pleasure of a woman's company. Plus..." Her lips pulled back into a smirk. "If it was Yukihime-sama, I'm certain she was very _generous_ with you."

A sudden blush rushed to the boy's cheeks. His mind went back to the events of tonight. But, soon shame turned into anger. That vile demon is a regular here? He disguises himself as a woman to defile these nice ladies? He preys on them like a wolf hiding in sheep's clothing! What if he was responsible for all of the missing courtesans!? _Usui-san is going to want to know about this!_

"Anyways, dear. We are a little short-handed tonight, so I was wondering if you could bring some of these things to Koinatsu-Oiran's room?" The courtesan pointed towards the pile of silks and chests next to her. "I know it's a lot, but we just don't have enough helpers tonight..."

Tanjirou shook his head, trying to shake away his anger and shame. "To Koinatsu-Oiran's room, right? Sure, I'll do it!"

The young slayer gave the courtesan a fake smile. He rushed to grab all of the fancy silks and heavy chests. Not wanting to keep talking about 'Yukihime-sama,' the young slayer rushed down the hall with all of the items in his arms. "Take a left at the end of the hall. Her room is the very last one!" The courtesan yelled as Tanjrou kept jogging away from her.

_ I need to get my mind off of this whole thing, and focus on the mission. Maybe I should report to Usui-san about this? _

"Sumiko." A monotone voice stopped him in his tracks. A strange scent struck him. His eyes followed the source.

A young, short woman stood in the middle of the large hall. Her black hair was shoulder-length and neatly cut. Her small frame looked so fragile and delicate. Anyone would probably mistake her for a harmless flower, but Tanjirou could tell that she was most _definitely_ not a normal girl. The way her piercing black eyes stared at him, like she hunting him. It was like staring into the eyes of a predator. Her scent was like that of a human, but at the same time it wasn't.

_Who is this girl? _

"W-Who are you?"

The girl ignored his question. Her delicate, bare feet made no noise as she approached him. "You need to be more careful. The longer you stay here, the better the chances that you will 'loose your footing.'"

Tanjirou instinctively backed away from her. Something about this woman made him feel unsettled. "What do you mean 'loose my footing'?"

The girl was now inches away from his face. He struggled to keep eye contact. Those eyes were like deep pools of emptiness. "Many courtesans have been 'loosing their footing; lately. It's what the locals refer to when one of these women decide to run away from this meaningless lifestyle. Once their caught, terrible things happen to them. Many of them don't come back."

"Don't come back..."

"Yes. Just like Suma-Oiran."

_Suma!? That's Usui-san's wife!_ "How do y-"

"Girls, would you please stop spreading such rumors." A kind voice interrupted them.

The two turned to see the beautiful Kotatsu-Oiran open her shoji doors. Her delicate face was shaped into an expression of discontent. "Really...There is no way to say for sure that those girls ran away. You mustn't spread such rumors without knowing the whole truth."

The strange girl besides Tanjirou did not respond to the older woman's words. Instead, she gave Tanjirou one last glance before she turned her back to the both of them. Tanjirou and the Oiran watched her calmly walk away.

"That girl is so strange..." The Oiran commented.

Tanjirou couldn't agree more. That girl is strange, and _dangerous_...

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Fuyuji walked away quickly, trying to get out of Tanjirou's sight as soon as possible.

The petite girl exited the brothel quietly. That's when she was bombarded with an annoyingly loud voice in her head.

_Fuyumi! What do you think you're doing!? Muzan-sama gave us direct orders to stop monitoring the Kamado siblings!_

Fuyumi flinched at Anne's volume. That blond woman never could learn how to keep her thoughts at a low volume. _What do you think I'm doing right now? I'm leaving this place. _

_The why, pray tell, are you speaking to that boy!? Why are you helping him find his missing comrade!? You need to get out of there now! One of the Upper Moons is in that brothel!_

Fuyumi scoffed. It's not like she didn't notice the demon's presence before. _I know... I could feel that putrid demon's presence the second it arrived here. _

_Lucy will be delivering the final ingredient to Tamayo tonight. We must get away from this mess before the real war begins… _

_I know, Anne... _

Fuyumi glanced behind her. _The bloodbath has hardly just begun..._


	5. Chapter 5

**Hey, guys! I hope everyone is healthy and staying safe! Please enjoy this next chapter. **

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Lucy hated being a messenger. She hated it even more when she had to deal with difficult recipients. The blonde, western beauty knocked the door, well actually wall, that led to Tamayo's home for what seemed like the tenth time. Clearly, she was being ignored and that was something the former lady of the night was not at all accustomed to.

"Hellooooooo, just how long do you intend to ignore me for, Miss High-and-Mighty Tamayo!?"

With one final knock, the secret 'door' opened, revealing a very irritated Yushiro. "How dare you insult Tamayo-sama, you English slut!"

Lucy's red lips pulled back in a smirk. "It's about time you answered me. Your Mistress is expecting me. I have a special package for her." She reached into her cleavage. Her hand pulled out a small vile with red liquid inside.

"That's it?" Yushiro asked with a disappointed frown. "Tamayo-sama was expecting more than just one little vile." The modified demon reached out to grab the vile, but Lucy moved it out of his reach. "What the hell is wrong with you!?"

"I'm sorry, dear. I have direct orders from my mistress to hand this little treasure over _only_ to Tamayo." She shook the bottle ever so slightly. "Can't risk this falling into the wrong hands..."

Yushiro could already feel the veins on his forehead begin to throb in anger. "Tamayo-sama does not need to be bothered with the likes of you! Now give me the-"

"Yushiro..." Tamayo's calm voice surfaced from behind him. Yushiro's sour demeanor immediately changed. Behind him, Tamayo's small frame appeared. Her lilac eyes bore straight into Lucy's blue ones. "Please...come inside, Lucy-san." The gentle demon retreated back into her home. Yushiro and Lucy followed her in. The two followed her into a dark room. Tamayo walked over to a large table and turned a lamp on. Yushiro quickly began to turn the other lamps on for her.

Lucy's eyes scanned the room. There room consisted of four metal tables, and two chairs. Each table was occupied with test tubes with various chemicals and lab was clearly a laboratory. Her eyes landed on Tamayo, who was patiently standing by one of her tables wearing her white lab coat. "My mistress told me that you were quite the scientists." Her hand reached for the vile. "I hope that means you know exactly what to do with this."

Lucy threw the vile towards them without much care. "You will _not_ be given more than this."

Yushiro quickly fumbled to catch the object before it hit the floor. "W-What is the matter with you!?" He glared at her venomously.

Tamayo grabbed the vile from Yushiro's hands. Her eyes scanned the contents. "I suppose this will have to do... but, with so little..." She sighed. "I suppose this means that there is little room for failure."

"There is actually _no_ room for failure." Lucy said in a serious tone. "Use the blood to complete whatever malicious drugs you're working on and use it to bring Kibutsuji Muzan down to his knees."

"Making him weak will not kill him." Tamayo said. "Someone needs to deal the finishing blow."

Lucy smirked. "Oh, you don't worry your pretty, little smart head about that. We already have a plan for that part..."

Tamayo narrowed her eyes at the Englishwoman. "I assume the Demon Slayer Corps will play a huge part in your plans?"

"But, of course." Lucy raised an eyebrow at the scowl that made it to the demon's face. "Oh don't give me that look! This is a game of chess, my dear! In chess there are always unsuspecting pawns who are put to use for the greater good." She began to approach the demons with an expressionless face. "Do you want to be rid of that monster or not?"

Tamayo lifted her eyes to meet Lucy's. "Yes. More than anything..."

A confident smile made it to Lucy's lips. "Good. Then you will do your part..." She turned around to leave. She stopped by the door frame. "...and leave the rest to us."

"Lucy-san..."

"Now, what?"

"I at least want to know one thing." Her eyes bore straight into the Englishwoman's back. "Why? Why after centuries of that woman being his _ally,_ would she want him dead now? For so long, she has always been his strongest supporter. I would have thought that hell would have frozen over before she ever raised a hand against him, but now... Why?"

"He crossed a line...One that he should have never touched..."

When the Englishwoman left, Tamayo placed a trembling hand on her forehead. Yushiro saw this and quickly turned to face her. "Tamayo-sama! Is something the matter?" His concerned-filled eyes searched for an answer from his mistress.

Tamayo stayed silent for a while before she lifted her face to give her ward a reassuring smile. "I'm alright, Yushiro. I'm just a little overwhelmed. For so long, I have longed to see an end to that monster...For so long, I have waited for a sign...a sign that finally that man's reign will soon come to an end...and this..." She lifted the vile up to her face. "This is my sign..." Her smile quickly turned into a frown when another thought came into her mind. "Although, I must say...I feel a bit guilty leaving the Demon Slayer Corps and the Kamado children in the dark about all of this."

"Tamayo-sama…"

* * *

Fuyumi had always been an obedient, dutiful servant to her mistress. Never has she ever gone against a direct order from her mistress. Yet, here she was defying an order to retreat. She should have disappeared with the others along time ago, but instead she stayed behind to watch the demon slayers fight against one of the weaker Upper Moons.

She honestly wished she could say she was impressed that a bunch of humans managed to kill and Upper Moon, but then she remembered that Daki and Gyuuntaro were the weakests of the Upper Moons. It wouldn't have even taken her and the others an hour to kill the two siblings, yet it took these humans an entire night to finish them off.

Fuyumi was assesing the damage done to the red light district, when she finally came across Kamado Tanjirou and his sister. The sister had finally gained back her senses whilst the older brother was still unconscious.

The petite girl watched with little interest as the Kamado boy finally woke up. She watched with a bored expression on her face as the boy was piggy-backed by his sister. They were heading towards the other young slayers. She was just about to consider leaving, when something interesting caught her eye. The Kamado girl, Nezuko, healed the boar-head guy with flames!

_Flames... I've never known a demon using their blood technique to heal others._ Her eyes widened when she saw Nezuko do it again. _This time to heal the Pillar. No, she's not healing him...she's purifying his body of the poison. No demon should be able to do that!_

With this final thought in mind, Fuyumi decided to finally retreat. She had to report this to her mistress.

* * *

**I know, I know...All of my chapters are kind of short, huh? I promise I will be writing longer chapters from now on. I just needed to establish the basic storyline with these short chapters. Let me know your theories and thoughts!**


	6. Chapter 6

**Hello, my beloved readers! Here is another chapter! Please, leave reviews. **

* * *

For centuries, the Ubuyashiki clan has fought to vanquish the demons that plague their country. Being the founders of the Demon Slayer Corps, it has been their eternal goal and burden to defeat the Demon King. Yet, for a hundred years, too many of their attempts to defeat him have failed miserably. Some were begging to believe that this war against demons was a one they were never going to win. But, something miraculous had occurred recently. Something that has not happened in a hundred years. The death of an Upper Moon! The defeat of a powerful demon related to Kibutsuji Muzan!

This news alone was enough to raise the spirits of many demon slayers. It especially caused a great stir within the Ubuyashiki household.

Ubuyashiki Kagaya, ninety-seventh head of the Demon Slayer Corps, had just risen from his sleep when his wife gave him the miraculous news. "What did you say!? An Upper Moon was defeated!?"

"Yes." His wife responded calmly. "The slayers responsible for this feat are all young slayers...Tanjirou, Tengen, Inosuke, Zenitsu, and the demon girl named Nezuko."

An excited smile made it to Kagaya's scared face. "I see, they really beat an Upper Moon... Amazing job, Tengen, Tanjirou, Neuko, Inosuke, Zenits-" His words were caught short as he doubled over coughing. Blood splattered on the tatami floor.

"Kagaya-sama! Please don't overexert yourself."

The two crows next to her cawed in agreement with her.

Kagaya ignored his wife's concerns. "For a hundred years! A situation that hasn't changed for a hundred years has finally changed!" He lifted his hea up, accepting the handkerchief his wife was offering. "Amane, do you know what this means?"

"Yes."

"This is an 'omen.' Fate is begging to change in a big way!" Kagaya had to pause for a moment in order to wheeze more oxygen into his weak lungs. "The ripple is begging to spread. It's will soon swallow everything in its path and cause a huge jolt. It won't be long before I will finally come face-to-face with that man...Kibutsuji Muzan...We will definitely defeat you in our generation! You are the single blemish on our entire family!"

"Kagaya-sama, there is one more thing. We received word from the demon Tamayo about the appearance of a new unlikely ally. One who may prove to be very useful..."

"What?"

"Her letter said that 'the dragon no longer protects Kibutsuji Muzan.'"

Kagaya's eyes widened in surpirse. "You mean to say that-" The sick man doubled over again to restrain his coughs. His lungs struggled to get air in. His wheezing and coughing echoed throughout the room.

"Father!" His daughters rushed into the room the second they heard their father struggling to breath.

"Dear! Kids, get some warm water and the medicine! Now!"

Kagaya could feel his wife and daughters near him, trying to ease his current suffering, but he paid no mind to them. Hid mind was plagued with only one thought.

_The dragon no longer protects Kibutsuji Muzan..._

_The dragon no longer protects Kibutsuji Muzan..._

_The dragon...is our ally? _

* * *

"Now, what's this?"

Muzan looked down on her subject. The petite kunoichi kneeled before her mistress. Her head lowered in submission. Fuyumi kept her gaze to the floor, attempting to avid eye-contact with anyone in the room. Her entire body was tense from her anxiety. Despite her mistresses' playful tone, she knew that her mistress was displeased with her.

"Fuyumi...my loyal servant...why are you so late to return home? Did you not receive the message to retreat?" Muzan spoke from her throne. She sat there in nothing but a black silk robe. Her legs and arms crossed as she fixed her red gaze on her smallest subject. Anne, her faithful shadow, stood beside her.

"Forgive me, Mistress..." Fuyumi spoke. "I did receive word of our retreat, but-"

"But what?" Her voice cut through the air like a knife. She rose from her gold and iron throne. She gracefully descended the steps to approach her subject. "What could have possibly gained your attention to stay behind?' She circled the girl like a predator, eyeing her the entire time. "What, pray tell, would ever compel you to disobey your mistress?"

Fuyumi swallowed loudly. "I wanted to watch the Kamado siblings fight the Upper Moon."

"Why? I thought I made it quite clear that I had no further interest in them."

"I wanted to asses their skills. I wanted to know if they would ever be a threat to yo-" Fuyumi gasped in pain when she felt a powerful hand grasp her face harshlyh. Suddnely, she was face-to-face with her mistress, staring into those ruby eyes. "Muzan-sama..."

"What was that you were going to say, Fuyumi? You wanted to know if those little brats would ever pose a threat to me?" Muzan dug her sharp nails into the kunoichi's soft skin, drawing blood. "Such foolishness! As if that man's children could ever scratch me, let alone threaten my life. The very notion is insulting."

"Please, Muzan-sama..." Fuyumi's eyes watered pleadingly, but she did not try to break her mistress' hold on her. "There is something you must know! The Kamado children killed the Upper Moon."

"So?"

"The girl...she is powerful, very strong, and...her Blood Technique...her flames can purify others..."

This caught Muzan's attention. She released Fuyumi immediately. "What? Kamado Nezuko can purify others with flames?"

"Yes...I witnessed her use it on a human. She purified his body of poison."

Muzan's eyes narrowed contemplatively. So, the purifying flame resides within that child now... Even though she is a demon... "Anne!"

"Yes, Muzan-sama?" Anne answered, still standing in the same spot beside her mistress' throne.

"Has Kamado Nezuko ever consumed human flesh? Has she ever been exposed to sunlight?" Muzan asked.

Anne closed her eyes briefly, attempting to recollect all information she had on the Kamado children. "To my knowledge, no. Kamado Nezuko has not once consumed human flesh, although she has come close to falling to temptaion... Instead of gaining strength through the consumption of human flesh, Kamado Nezuko gains her strength through sleep. She has never once been exposed to sunlight...I suspect her brother has made sure not to take the risk of allowing her to anywhere near sunlight."

Muzan scowled. "I see...I suppose that is why the child an wield something as powerful as the Flame of Purity. But, for a demon to wield that power... I always thought it to be impossible... Have I underestimated these Kamados?" She subconsciously lifted her hand to touch her lower stomach. "I do not like this..."

"Muzan-sama?"

Silence filled the room. Muzan turned her head away from her subjects. Her expression turned dark. Her mind reeling with concerned thoughts. The Flame of Purity within that man's child... Not to mention, the tow of them seem strong. Growing stronger by the day... If they ever become string enough to confront him directly, will they learn the truth? What will they do if the past is revealed?

The large double doors to the left-side of the room suddenly opened. Every person in the room turned their gazes towards the new arrival. Lucy took a good, long look at the room and huffed. "Now, what's all this? I leave for a little while and suddenly the whole place is so gloomy? You're all acting like someone just died."

Muzan's scowl turned into a mischievous smirk. "Not yet, my sweet Lucy!" Her eyes followed the Englishwoman carefully. Lucy approached her mistress with a curtsy. Lifting Muzan's pale hand to her lips, she gave it a kiss. "I hope you had no trouble delivering my gift?"

Lucy lifted her head, showing off her gleeful smirk. "But, of course not, Muzan-sama! Tamayo was quite pleased with your gift. I'm sure the Demon Slayers will be just as pleased."

Muzan nodded in approval. "Good! All is going according to plan, but one other thing, ladies..." The demoness turned to address her other subjects. "I've decided to keep a closer eye on these Kamado children. I had every intention of leaving out of my plans, but I can't bring my child into this world with all these uncertainties. I don't care how many demons and slayers die at the end of this war, I just want to make sure that those two are gone by the time this child is born." Her hands stroked her lower stomach. She strides over to her throne. "Fuyumi, you will continue to monitor these young slayers, making sure to keep a close eye on the Kamado siblings."

"Yes, Muzan-sama!"

Muzan sat comfortably on her large throne. "Lucy, you will keep a vigilant eye on Tamayo's progress. I want to know exactly how she is using my blood and what for. She too is a loose stri ng I intend on getting rid of."

"As you wish, Mistress."

"Anne, call a meeting. I need all of my beloved subjects to be aware of my plans..."

"Understood, my Queen."

* * *

The dragon no longer protects Kibutsuji Muzan...

What does this mean for us? What does it mean for the Slayers? If the dragon is no longer on his side, then what does that mean? Is she our ally now? What are her intentions? Why has she forsaken Muzan?

Ubuyashiki Kagaya sat comfortably on a bench in his garden. His eyes were watching the Koi fish swim about in his pond, but his mind was consumed with thoughts from this morning's news. Nothing else has occupied his thoughts since. It was begging to concern his family even more than they usually are. He needs to know the answers to his questions. He needs know where the dragon stands, now that she won't protect the demon king. He needs to reach out to Tamayo. No...Tamayo-san isn't at all close with that person. I need to confirm this from someone more reliable.

Kagaya's thoughts were interrupted when he felt a new presence enter his garden. "It has been a while since anyone has been able to locate my hiding spot. Should I be concerned for my wellbeing?" He spoke to the intruder.

The intruder leaned against a cherry blossom tree. "The elves are not concerned with your secrets, human. Don't flatter yourself."

"Then, why are you here?"

The intruder huffed. "We know you have valuable information concerning the demon king and the dragon. Tell me know, human."

Kagaya couldn't resist the smirk that made it to his lips. "For centuries the elf clans have refused to aid our cause to end Muzan's reign. Even though we have always had the same goal in mind, you elves have never wanted to join forces with us. Now that there is progress on our end, you want me to cooperate? Seems a bit selfish of your clan leader to demand this of me, a human."

The intruder growled. He moved from his spot by the tree to come face-to-face with Kagaya, finally revealing himself. Blue eyes glared into hollow, purple ones. "This is no time for your games, human! You know perfectly well why we will never work with you filthy humans again! The last time we joined forces with you, it gave birth to a horrible monster!" The male elf grabbed Kagaya's neckline. The sound of crows cawing maniacally filled the air. It would not be long before someone came to see what all he ruckus was about. "Tell me what you have discovered! We know that one of your slayers came in contact with the dragon!"

Kagaya stared at his aggressor with a completely calm demeanor. "All I know is what Tamayo-san said to me, that 'the dragon no longer protects Kibutsuji Muzan.' As for this 'meeting' between one of my children and the dragon, I'm afraid I don't have the slightest idea of what you are talking about." His answer was followed with several painfully bloody coughs.

The elf cringed at the sight of blood dripping from Kagaya's mouth. "How disgusting. This is the curse that has plagued your family since the begging of this war, huh? I almost feel sorry for you, Ubuyashiki. But it serves you right! It was your ancestors that created those monsters..." He released Kagaya from his grasp. "So, the dragon has finally forsaken that demon... The elves will be interested to hear about this. Yet, I can't help but wonder why... Are you certain you know nothing of the dragon's interaction with one of your slayers? Tell me the truth, human."

Kagaya took a deep breath before answering. "Unfortunately, I don't. If I had any idea of one of my children coming that close to-" Kagaya suddenly paused. A certain young demon slayer came into mind. "Unless it was..."

"Unless it was who?"

"I'm afraid I cannot say..." Kagaya said in a serious tone. "You'll have to forgive me, Elf-san, but as you and I have already sated, humans and elves are not fighting together to eradicate our mutual enemy. Therefore..." He raised himself from his bench. "I cannot just give you the name of one of my children. But, you can rest assure that I will look into this matter myself. So, if you would please leave?"

The elf intensified his glare. "You selfish human! You are truly a creature deserving of your family's fate! Do not think this is the end of the matter!"

"Oyakata-sama! What was noise!?" The voices of his beloved 'children' alerted the elf.

Wanting to avoid having to deal with more humans, the elf decided to make his leave. "I will not forget this, Ubuyashiki!" He ran towards the manor walls.' Leaping over the stone structures, the mythical creature disappeared.

The dragon no longer protects Kibutsuji Muzan...

That was a fact Kagaya was begging to come to terms with, but now there was something else bothering him. What does Kamado Tanjirou have to do with all of this?


End file.
